An optical processor, such as a two-dimensional spacial light modulator, can compare an input pattern with a Fourier transform of a template pattern to determine their degree of correspondence. Most prior art optical processors involve the comparison of a pattern formed on a photographic film optical transparency with the Fourier transform of a template pattern which is formed on another photographic film. This has the disadvantage that a transparency photograph has to be produced and developed each time an input image is to be compared to the template image. One variation of this, described in U.S. Pat. No. 4,018,509 by Boswell, is to focus an image of a transparency onto an array of photoconductors and liquid crystal pixels, to control the reflectivity of the liquid crystal array. In addition to requiring a transparency, the system is expensive, in that it is expensive to construct a combined array of photoconductors and liquid crystal pixels. One system suggested by Hughes Aircraft Co. uses charge coupled devices controllable by video signals, to control the reflectivity of crystal liquid devices. The production of the array of charge couple devices used in such a system is very expensive. An optical processor which could construct an input pattern for comparison with the Fourier transform of a template pattern, which enabled the rapid and inexpensive creation of input patterns in an input device of low cost, would be of considerable value.